This invention has relation to apparatus and methods for mixing different fuel gases with themselves and/or with air to produce fuel gas mixtures which will have uniform and efficient burning characteristics in gas appliances without any change in adjustment of the burners on those appliances. Among the factors which affect the satisfactory interchangeability of fuel gases are the flame characteristics of different gases and mixtures of gases. These characteristics include the tendencies for lifting, yellow tipping, and flashback. It has been discovered that if a heating value equivalency in the various mixtures of gases fed to gas appliances can be maintained relatively close to the heating value equivalency of a gas on which initial adjustment of the gas burners has been made, satisfactory results can be obtained without the necessity of readjusting the burners as other changes occur in the make-up of the gas mixtures fed to the appliances.
Such a heating value equivalency can be defined as the heating value of a gas in heat units per unit of volume divided by the square root of the specific gravity of the gas. Initially in the British practice, this was called the Wobbe Index or the Wobbe Number. The heating values of various gases can be determined from the specific gravity of those gases so a measurement of the specific gravity of a particular gas mixture allows the heating value of that mixture to be calculated.
It is known to calculate the Wobbe Number or heating value equivalency for natural gas, for example, under conditions of standard pressure and temperature; and then to calculate the ratio of a liquid petroleum gas such as propane to be mixed with air at standard pressure and temperature in order to achieve a mixture having substantially the same heating value equivalency or Wobbe Index as that of the natural gas.
Then, taking into consideration an assumed propane temperature and air temperature and an assumed pressure in the propane and air lines, it is possible to calculate the flow rate needed in each of those lines to achieve the heating value equivalency or Wobbe Index of natural gas in the resulting propane/air mixture. By accurately controlling such a flow rate and by accurately maintaining the temperatures and pressures at the assumed values, and assuming the specific gravity of the propane to be uniform and unchanging, a propane/air mixture having the same heating value equivalency as that of the natural gas can be produced.
In a dynamic system, however, the demand for effluent or output gas changes, the supply of propane and/or natural gas varies causing pressure changes in the supply lines, and the ambient or input temperature of each gas and of the input air changes. Also, where off gases from other processes are to be combined with other fuel gases, the specific gravity and heating value is a variable. As each of these factors changes, new calculations are needed to determine the flow rates and the ratio of flow rates of fuel gases and/or air needed to continue to achieve in the fuel gas output mixture the heating value equivalency or Wobbe Index of the natural gas or other gas for which the burners of the gas appliances were initially adjusted. After and as these calculations are made, instantaneous and continuously changing adjustments of the flow rates would have to be achieved. This was not possible prior to the present invention.
Therefore, a control system was needed to automatically take into account these changes in make-up of the gases to be mixed, changes in demand, in input pressures, and variations in specific gravity and temperature. The apparatus and control system of the present invention was developed to meet that need.
A preliminary search was made on this invention, and the following patents were identified:
U.S. Pat. No. 3,383,190 to Weber et al, granted in May of 1968; PA1 U.S. Pat. No. 3,433,606 to Moore et al, granted in March of 1969; PA1 U.S. Pat. No. 3,491,585 to Hass, granted in January of 1970; PA1 U.S. Pat. No. 3,503,722 to Weber et al, granted in March of 1970; PA1 U.S. Pat. No. 3,582,281 to Fenske et al, granted in June of 1971; and PA1 U.S. Pat. No. 3,999,959 to Bajek, granted in December of 1976.
These patents are merely illustrative of the type of subject matter which is patentable and are not believed to be particularly pertinent to the present invention. Applicant and those in privity with him know of no closer prior art than that set out or discussed above; and they know of no prior art which anticipates the claims made in this application.